This is likely to be a matter of policy rather than law
That is, it’s not likely there is a law prohibiting it but it is highly likely that the person’s training and their employer’s policy on the matter is that they must complete a ticket once they start it.
It’s a pretty universal anti-corruption measure - it prevents the situation where they are writing ...
From your link
EXTENUATING CIRCUMSTANCES - a special or specified circumstance, including
such types of extenuating circumstances established by the City Solicitor, that partially or
fully exempts a person from performance of a legal obligation so as to avoid an
unreasonable or disproportionate burden or obstacle.